CDKL5 is an X linked gene, when mutated causes a neurodevelopmental disorder with the same name, generally categorized as an atypical Rett syndrome. Despite general similarities with Rett syndrome, early onset seizures, typical of CDKL5 deficiency, is not observed in Rett. CDKL5 mutations are often loss of function alleles resulting in a mosaic expression of CDKL5 in female patients. Missense mutations are also identified in the kinase domain, indicating that the kinase function is critical. Males CDKL5 patients are also found, albeit less often. Several treatment options are being evaluated at the moment ranging from protein/ gene replacement to stop codon read-through therapies. Biomarkers which could be used to measure recovery of CDKL5 activity would be helpful in evaluating the outcomes in animal models and as well as in patients. We propose in this study to test if phospho-specific antibodies raised against CDKL5 substrates that we have discovered could be used as biomarkers in CDKL5 knockout mice, patient neuronal cell lines and patient samples.